1. Field of Art
This invention relates to systems which induce feel-force in aircraft control sticks by means of hydraulic pressure, and more particularly to improvements in detecting faults in the operation of such systems.
2. Description of the Prior Art
For some time, it has been known to utilize hydraulic actuators connected to the linkage of an aircraft control stick to provide force to the control stick which is dependent in some fashion upon the position of the stick, to indicate to the pilot the degree of command caused by him, which in turn is an indication of the loading of the aircraft surfaces.
Because there is a possibility that such a hydraulic actuator can move the control mechanisms accidently, in a fashion to cause undesirable or catastrophic flight maneuvering, it has been known to provide fault detection apparatus for hydraulic feel-force systems. One fault detection apparatus known to the prior art compares the force output of two actuators in a pair of completely separate force augmentation systems; divergence between the two, in excess of a threshold magnitude, indicates failure. However, such apparatus is extremely costly and excessively heavy, to a degree which may be unacceptable. A second fault detection apparatus simply monitors the force-related pressure of the actuator, and if it exceeds a given magnitude for a certain period of time, the system is shut down. However, this system is highly susceptible to nuisance shutdown as a consequence of abrupt pilot-induced motion to the stick causing high pressure differences across the actuator. And, the sophisticated modifications added to such system, to cause it to operate in an acceptable manner, provide cost and weight penalties similar to those of the system described hereinbefore.
To overcome these difficulties, a fault detection system having a certain degree of direction sensitivity has been devised. In this system, stick motion in the same direction as an excessive pressure (proportional to force) error in the force actuator is monitored, and when such stick motion exceeds a threshold value, the system is shut down. This system is described fully in the detailed description, hereinafter. This system has fewer, less significant problems than those described hereinbefore. But, it requires a balance between nuisance shutdowns as a consequence of long term drift on the one hand, and insensitivity to slow, fault-generated stick motions which are masked by drift compensation, on the other hand. Also, the threshold detecting of a magnitude of error indicative of fault is done in a single polarity domain (positive or negative) following integration of signals in that single domain, based either on non-inverted signals of one polarity or inverted signals of another polarity. Thus the thresholding to detect fault is not directionally sensitive. Because this detecting apparatus utilizes nonlinear integration following differentiation, unequal integration of noise can result in nuisance shutdowns and other problems. The circuitry necessary to actually implement such an apparatus in the technological real world becomes expensive and complex, as well. These factors are discussed more fully following a general description of this prior art fault detection apparatus, in the detailed description, hereinafter.